THE OVUM. 



47 



The ovaries are paired organs, rarely directly connected, 

 rarh consisting of more or fewer ovarian tubes which open into 

 a common oviduct. The oviducts unite into a vagina, usually 

 provided with a spermatheca and accessory glands, which need 

 not be further alluded to. Each ovary is invested by a peri- 

 toneal covering, which assumes various characters, and either 

 forms a loose network covering the whole or a special tunic 

 round each egg-tube. It is continuous with the general peri- 

 toneal investment. Each ovarian tube (fig. 17) consists of three 

 sections: (i) a terminal thread, 

 (2) the terminal chamber or ger- 

 mogen, (3) the egg-tube proper. 



The whole egg-tube is invested 

 in a structureless tunica propria. 



The terminal threads are fine prolon- 

 gations of the ends of the egg-tubes usually 

 continued close up to the heart. At their 

 extremities they frequently anastomose, 

 or even unite into a common thread. In 

 some cases they are absent. They form 

 either direct continuations of the ger- 

 mogcn and have the same histological 

 structure, or in other cases are simply 

 prolongations of the tunica propria, and 

 serve as ligaments. 



The germogen usually consists 

 of two parts : an upper, filled with 

 nuclei imbedded in protoplasm, 

 and a lower, in which distinct cells 

 have become differentiated. 



The lower part of the egg-tubes 

 is filled with ova which advance in 

 development towards the oviduct, 

 and lie in chambers more or less 

 di>tinctly constricted from each 

 other. In these chambers there 

 arc in most forms in addition to 

 the true ova a certain number of 

 nutritive cells. The true egg-tubes 

 .nx moreover lined by an epithe- 



Fio. 17. A . OVARIAN TUBE OF THE 

 FLEA, PULEX IRRITANS. (From 

 Gcgcnbaur, after Lubbock.) 

 o. ovum. g. germinal vesicle. 

 B. OVARIAN TUBE or A BEETLE, 

 CARABUS VIOLACEUS. (After Lub- 

 bock.) 



o. ovarian segment, formed of an 

 ovum a. and a man of yolk-crib, *. 



